This worked for me, thanks for the tip Jonathan. We've got a STX1-4 and it was giving the old E8 error too.
I turned off the tap and power, pulled the dishwasher out from under the counter, and laid it gently on its side. Then I opened the plastic panel at the bottom of the dishwasher (requires 2 screwdrivers to lever open the clasps) and tried to locate the diverter valve. If you don't recognise it immediately, you can work out which component it is by following the hoses - the water first comes into the machine in the hose from the tap, which goes into a pump. Then there's a single hose from the pump to the diverter valve. The diverter valve has two water outputs - one into the base of the machine (next to the filter) which is a white plastic connection, and the other a hose which goes up the back of the dishwasher to deliver water to the rotor arm under the top basket.
I disconnected the electrical supply from the diverter valve (be careful not to yank any wires when you disconnect the electrical connectors from each other), and took off the hoses and removed the valve from the machine by unscrewing the single screw. Then I opened it up on the table and found it was full of scale/gunk. I took it apart and, with a tiny screwdriver and some silicon spray, removed all the scale and got the various plastic cogs etc rotating smoothly. Then I put it all back together (don't forget to put the rubber hood which goes over the electric motor back on!) and reinstalled it in the machine (again, be careful not to yank the wires).
After I turned the mains and the water tap back on I gave it a test run - worked like a dream and has run 4 loads since with no problems.
Sam Critchley, July 2011